The present invention relates to a method of magnetic resonance imaging, or more in particular to a method of magnetic resonance imaging which makes it possible to prepare tomographic images of a plurality of faults in a subject at a time.
The operating principle of a magnetic resonance imaging apparatus will be generally explained. The magnetic resonance imaging apparatus is for exciting the phenomenon of magnetic resonance in a specific fault selected in a subject or specimen and imaging the resulting resonance signal as a fault plane information. The condition for generating a magnetic resonance is given as EQU f=.gamma..multidot.H (1)
where f is the frequency of a high-frequency magnetic field applied for exciting a magnetic resonance, .gamma. the gyromagnetic ratio (a constant determined by nuclear species) and H the intensity of the magnetic field.
In order to excite the phenomenon of magnetic resonance in a subject, a high-frequency magnetic field having a frequency meeting the above-mentioned condition is applied. Conversely, as far as the above-mentioned condition fails to be met, no magnetic resonance is attained even when a high-frequency magnetic field is applied.
An imaging technique is well-known and mainly utilizes the two-dimensional Fourier method.
If an imaging is to be conducted successfully, it is necessary to generate a magnetic resonance in the spin of the fault intended for imaging. Generation of a magnetic resonance in the fault is called "slicing".
The principle of slicing will be explained with reference to FIG. 1. In FIG. 1, an example for slicing along a cross section of the head of human being, i.e., a plane crossing at right angle to the axon (Z axis) is illustrated. According to a method of magnetic resonance imaging, the subject is placed in a static magnetic field of uniform intensity, and a magnetic field different in intensity with the position along the axon crossing at right angle to the fault plane intended for slicing is superimposed on the uniform static magnetic field. The magnetic field to be superimposed on is generally the one of which the intensity of the magnetic field changes linearly with the positional coordinate. A magnetic field such as this is called "an inclined magnetic field". In FIG. 1, reference numeral 101 designates a head, and numeral 102 a fault to be imaged. The graph 3 plotted under the head shows a change characteristic of an inclined magnetic field. In the coordinate of the graph, the abscissa represents the position along the direction of axon (Z axis), and the ordinate the intensity of the magnetic field. A position in the head 101 corresponds to the one along the abscissa of the graph 103. In FIG. 1, the intensity of the magnetic field corresponding to the central position 102a of the fault 102 is expressed as H'. Substituting this intensity of magnetic field H' into the equation (1) representing the resonance condition, EQU f'=.gamma..multidot.H'
This shows that the frequency of a high-frequency magnetic field required for generating a magnetic resonance in the fault 102 is give as f'. Actually, however, the fault 102 has a predetermined thickness as shown and the value f' has an expansion corresponding to the thickness. Therefore, the frequency component of the high-frequency magnetic field is required to have a uniform intensity within the bandwidth of EQU f'-.DELTA.f.about.f'+.DELTA.f
In order to apply a high-frequency magnetic field as described above to the subject, the subject is impressed with a high-frequency magnetic field obtained by subjecting a carrier wave 105 of frequency f' to amplitude modulation in a waveform based on a sinc function 104 as shown in FIG. 2.
Completion of slicing also ends the application of an inclined magnetic field. At the end of slicing, therefore, the subject is placed again in a uniform static magnetic field. As a result, the resonance frequency in the subject changes from f' to f.sub.0. This value f.sub.0 represents a resonance frequency corresponding to the intensity of a uniform static magnetic field H.sub.0.
In other words, all the spins excited tend to return to the original state while rotating at the resonance frequency f.sub.0.
Then, in order to identify each resonance signal from the fault 102 in accordance with the position thereof, two coordinate axes intersecting each other at the end of the fault are set, and a signal is taken in with the frequency and phase of the resonance signal set in correspondence with each other along the direction of each coordinate axis. The signal thus taken in is subjected to Fourier transformation, separated by spatial position and is arranged on a screen corresponding to each position thereby to obtain a fault image. Setting a positional coordinate in correspondence with the phase and frequency of a resonance signal is called "the phase encoding" and "the frequency encoding" respectively.
The above-mentioned imaging technique is called the two-dimensional Fourier transformation imaging. A general timing chart for this technique is shown in FIG. 3. In FIG. 3, the abscissa represents a time axis, Gs an inclined magnetic field for slicing used to select a fault wherefrom a resonance signal is to be taken out, Gp an inclined magnetic field for phase encoding which is maintained at a predetermined magnitude during a series of operations for taking in a resonance signal, and Gf an inclined magnetic field for frequency encoding applied when taking in a resonance signal. The ordinate of each graph represents the magnitude of inclination. The inclined magnetic field Gp for phase encoding is changed in magnitude by a predetermined value for each series of operation. The resonance signal 106 taken in is subjected to homodyne detection by a reference signal having a reference frequency so that the resonance signal is changed to a signal having a frequency equivalent to the difference with the reference frequency. Further, the signal data 107 thus obtained is subjected to two-dimensional Fourier transformation thereby to prepare a fault image.
Another method of producing a fault image is called the three-dimensional Fourier transformation imaging. According to this method, a plurality of successive faults at predetermined intervals along the direction of slicing (direction of axon) are excited by a high-frequency magnetic field of the same frequency at a time as shown in FIG. 4A. A related pulse sequence is shown in FIG. 4B.
In order to discriminate a plurality of slice sections from each other, a phase encoding pulse is applied also along the direction of slicing thereby to differentiate the rotational phase of spin for each slice section. The three-dimensional imaging, though superior in photographing successive adjacent faults, requires a measurement time and data processing more than necessary when it is desired to obtain only images of a plurality of faults distant from each other in a three-dimensional photographic block, thereby leading to the disadvantage of considerable waste. This problem is illustrated in FIG. 5. In FIG. 5, in the case where it is desired to obtain fault images of four faults A to D distant from each other, a slicing operation is required as specified by the minimum slice width and position. It is, therefore, necessary to produce images of the four faults and also the remaining two successive faults adjacent to the respective faults. This poses the problem of necessity for taking measurements four times as much as is originally necessary for the faults A to D.
A technical reference on the prior art related to the present invention includes "POMP (Phase Offset Multi-Planar) ", by G. H. Glover and A. Shimakawa General Electric Medical Systems, Collection of Texts Vol. 1, 1988, for The Seventh Annual Conference of Society of Magnetic Resonance in Medicine. According to the cited reference, in preparing a plurality of fault images, a high-frequency magnetic field irradiated on a subject for selecting a fault is synthesized by a plurality of high-frequency magnetic fields different in frequency corresponding to each slice section. When the high-frequency magnetic field is removed, the spins of the excited slice sections are rotated with the same frequency. In order to obtain information along the direction of axon in each slice section, the conventional method described in the cited reference discloses a technique in which the phases of the synthesized high-frequency magnetic field are displaced by a predetermined interval without applying a phase encode pulse along the direction of slicing, so that the rotational phase of the spin of each slice section is differentiated simultaneously with the excitement of the slice section thereby to select a plurality of faults. This technique requires a technologically complicated, sophisticated phase control.
An example of a high-frequency magnetic field synthesized as above is shown in FIG. 6. As seen from FIG. 6, the phases .theta..sub.1 to .theta..sub.4 are required to be changed by a predetermined amount each time a high-frequency magnetic field is applied.
The difference in phase change is detected as a frequency difference by Fourier transformation thereby to produce a signal associated with each slice section.